There's a great story about why the sky is blue during the day, and turns golden during sunsets:
Rayleigh scattering affects blue light more. During the day, blue light from the Sun is scattered towards us from all directions, causing a blue sky. During the sunset, the length of atmosphere the light from the Sun has to travel through becomes so long that the blue is depleted, giving the sky a golden color.
However, it seems this can't be the full story, because photographers know that after sunset, there is a so-called blue hour where the color of the sky becomes a deep blue again. Why would the color go from blue to golden to blue again?
Wikipedia states very strongly that explanations of this in terms of Rayleigh scattering are wrong, and that the real explanation is the absorption of blue light by ozone. But it doesn't explain why that would create the effect. If the blue light is not reemitted, then this just amounts to having less blue light, so it can't explain why the blue hour is more blue. And if the blue light is reemitted, then the effect of ozone should qualitatively be very similar to the effect of Rayleigh scattering, since it's just another scattering route that favors blue light, leading us back to the original puzzle.
What's the right explanation for the blue hour?